a) Field of the Invention
The invention is directed to a method for electronic image rectification in pixel-by-pixel scanning of a substrate by means of a bundled electromagnetic beam, especially a laser beam, in which the pixels are read out successively during associated readout intervals of constant duration. The invention is further directed to laser scanning devices such as laser scanning microscopes, chip readers and the like, with scanning light points.
b) Description of the Related Art
Methods of the type mentioned above are suitable particularly for image data generation for electronic image analysis. A substrate is resolved into a multitude of pixels which are scanned, usually line by line. Optical scanning has proven successful for scanning extended object fields because of the higher scan speed. Therefore, it is preferable to use scanning devices in which an illuminating light point scans over the substrate through the use of oscillating mirrors, particularly galvanometer mirrors or rotating polygon prism mirrors. The light influenced by the substrate is detected by a detection device and evaluated. The image information obtained in this way is subsequently stored by pixel. When all of the information associated with the individual pixels is put together, a total image can be generated which can be analyzed or processed further by image processing methods which are known per se.
Because of the pixel-type construction, the quality of the total image obtained is highly dependent on the reproducibility of the geometric ratios of the substrate in the image. This is primarily a matter of correlating the image information read out by the detection device to spatially defined pixels. It is immediately apparent that interference can occur in the correlation due to variations or deviations in a predetermined reference scanning movement. For example, when the image is divided into parcels by the subsequent steps and a “measurement” is generated from the pixels of a parcel, distortion of the image can cause an erroneous correlation and accordingly an erroneous measurement. Image rectification by software prior to further image processing steps requires more computing time and can lead to increased crosstalk between the “measurements” of adjacent parcels. Preventing such artifacts requires time-consuming algorithms which likewise increase the computing time for image processing in some cases.
For this reason, efforts have been made in further developments to actualize a predetermined reference scanning movement in the most precise manner possible, which involves minimizing deviations between the position of the laser beam during acquisition of a pixel value and the ideal reference position as far as possible. However, with the forward feed devices which are usually used for this purpose, precise scanning movements can be realized only at a very high cost in apparatus. Moreover, a systematic residual error between the reference scanning movement and an actually performed scanning movement can also not be entirely eliminated in this case. These deviations are noticeable as distortions in the total image composed of the pixels.
Another possibility for improving the image quality consists in electronically reworking the total image obtained from the scanning process. However, as was already mentioned, the expenditure entailed by a reworking of this kind is considerable and it is therefore undesirable in practice.